Alkali flame ionization detector having cap means for changing the gas flow pattern

ABSTRACT

A hollow flame jet is supported on a base member and surrounded by a cylindrical chimney. The outlet end of a chromatographic column extends into a mixing chamber in the base member where the effluent vapors from the column are mixed with hydrogen. This mixture issues from the flame jet and is burned in the presence of an alkali salt contained in a cup surrounding the tip of the jet. Air to support this combustion enters the chimney at the base of the flame jet. A pair of electrodes positioned near the jet are connected to a fixed voltage power supply and electrometer circuit in order to cause a current to flow in the interelectrode space and to measure this current. The magnitude of this current is proportional to the density of ions in the interelectrode space. Since nitrogen phosphorus-, sulfur-, and chlorine-containing hydrocarbon compounds produce relatively high currents compared with other hydrocarbon compounds, they can be reliably detected and quantified in the effluent stream.

United States Patent [72] Inventor Charles H. Hartmann PrimaryExaminer-Morris O. Wolk 1048 Sanders Drive, Moraga, Calif. 94556Assistant Examiner-R. M. Reese [2]] App]. No. 836,350 Attorneys-StanleyZ. Cole and Leon F. Herbert [22] Filed June 25, 1969 [45] Patented Sept.21, 1971 ABSTRACT: A hollow flame jet is supported on a base member andsurrounded by a cylindrical chimney. The outlet end of a chromatographiccolumn extends into a mixing [54] ALKALI FLAME IONIZATION DETECTORchamber in the base member where the effluent vapors from HAVING CAPMEANS FOR CHANGING THE GAS the column are mixed with hydrogen. Thismixture issues from FLOW PATTERN the flame jet and is burned in thepresence of an alkali salt 5 claimsz Drawing Figs contained in a cupsurrounding the tip of the jet. Air to support this combustion entersthe chimney at the base of the [52] US. Cl 23/254 E, flame jet- A pairof electrodes positioned near the jet are com 23/232 C nected to a fixedvoltage power supply and electrometer cir- [51 Int. Cl ..G0ln 31/12 cuitin Order to cause a curl-em to flow in the interelecn-ode [50] Field ofSearch 23/254, Space and to measure this curl-em The magnitude f thiscur 254 1 ,2 rent is proportional to the density ofions in theinterelectrode space. Since nitrogen phosphorus-, sulfur-,andchlorine-con- [56] Reerences cued taining hydrocarbon compoundsproduce relatively high cur- UNITED STATES PATENTS rents compared withother hydrocarbon compounds, they can 3,423,181 1/1969 Dimick et al.23/254 be reliabl detected and uantifled in the effluent stream. E 3' qRRRR 65 souRcE POWER SUPPLY A 65 ATTENUATOR 59 ,7

I RANGE RECORDER 57 55 I SWITCH 69 7| -55 I 15 T 42 54 ll HYDROGEN mPATENTED sERzT RN CURRENT SOURCE BUCKTNG R 2 FIG. 2 j? 28 59 55 57He=|9m|/min 25 go "L I I 3: 6- n, 83. l/ I I l5 5 L T% 42 54 N EHYDROGEN IN g BACKGROUND ouRRENT Ys Ns TNNYTAzoBENzENa g RESPONSECHARLES T'TT YT KRRNRNN n p B HYDROGEN FLOW m|/min vim 7-7 W ATTORNEYALKALI FLAME IONIZATION DETECTOR HAVING CAP MEANS FOR CHANGING THE GASFLOW PATTERN Unlike earlier detectors which permitted virtuallyunrestricted flow of air and combustion products out the top of thechimney, the detector described is provided with an airtight cap. Thusair entering the base of the chimney, instead of sweeping upwardly in alarge amounts past the combustion region, forms a relatively static poolaround the lower end of the flame jet. From this pool air sufficient tosupport com bustion diffuses upwardly to the combustion region where itis consumed. The products of combustion together with air in excess ofthat needed for combustion exits at the unsealed joint around the baseof the chimney.

BACKGROUND OF THE INVENTION The present invention relates generally tothermionic detectors used in gas chromatographic analysis and inparticular to such detectors equipped with a source of alkali saltadjacent the combustion region. Such detectors, known as alkali flameionization detectors or AFID's, are especially useful in determining thepresence and concentration of phosphorous, nitrogen-, sulfur-andchlorine-containing hydrocarbon compounds in the effluent stream from agas chromatographic column because they exhibit enchanced sensitivity tothese compounds as compared to other hydrocarbons present in theeffluent stream. For a discussion of such detectors reference may bemade to the following: U.S. Pat. No. 3,423,181, issued Jan. 21, 1969, anarticle in Nature," 201, 1,204 (Mar. 21, 1964) by Karmen and Guiffrida,article in Analytical Chemistry, 36 1,416 (July 1964) by Karmen.

In general an AF ID detector provides means for mixing the effluentgases from a chromatographic column with a combustible gas such ashydrogen and burning the mixture in the presence of one of the salts ofan alkali or alkaline earth metal. A pair of electrodes disposed in thevicinity of the flame are connected to a constant voltage DC source andthe ionic current flowing between the electrodes is measured with anelectrometer and plotted as a function of time. The flame jet andelectrodes are typically enclosed within a cylindrical tower or chimneywhich shields the combustion region from the ambient atmosphere. Air tosupport combustion is introduced under pressure at the base of thechimney and flows upwardly to the combustion region and out the topwhich is typically partially closed by a loosely fitted lid whichrestricts the flow of ambient air to the combustion region.

For reasons that are not clearly understood the products of combustionof nitrogen-, phosphorous-, sulfur-, and chlorinecontaining hydrocarboncompounds cause a dramatic change in the ionic density and hence in thecurrent in the interelectrode space. Typically the current increases inthe presence of one of these compounds. However sulfur and chlorinesometimes cause a decrease. The alkali flame ionization detector (AFID)exhibits a markedly enhanced sensitivity to the nitrogen-, phosphorus-,sulfurand chlorine-containing hydrocarbon compounds in comparison withthe sensitivity to other hydrocarbon compounds. Thus, in theory, verysmall concentrations of these compounds can be measured in the effluentof the chromatographic column even when the effluent containssubstantial concentrations of other hydrocarbon compounds.

Unfortunately the sensitivities which have been realized in practicehave not been as high as expected largely because of excessive noise. Inaddition the background current, which is the current existing underquiescent conditions when no sample is present, has been excessivelyhigh when the air and hydrogen flows were properly adjusted to giveadequate sensitivity and selectivity for the hydrocarbon compounds ofinterest.

Since it appeared that the flow pattern of air within the detectorchimney might be responsible for some of the noise encountered withprior art detectors work on controlling the flow pattern was commenced,resulting in the present invention.

SUMMARY OF THE INVENTION According to the present invention the flow ofair within the detector can be controlled in such a way as to reducebackground current and noise while enhancing sensitivity to thecompounds under investigation by providing a removable sealed top forthe chimney and an exhaust passage for the products of combustion andfor excess air at the base of the chimney. Thus in operation the airinstead of flowing in quantity past the combustion region and out thetop, diffuses from the base region of the chimney into the combustionregion only in sufficient quantities to be virtually consumed by theflame. Any excess air is exhausted together with combustion productsthrough the exhaust passage near the base of the chimney. The result isthat the deleterious effects of the unrestricted upward flow of air inthe prior art detectors are substantially eliminated.

The principal object of the present invention is to provide an improvedalkali flame ionization detector having enhanced sensitivity to organicvapors of compounds of interest while substantially reducing backgroundcurrent and noise.

Another object is to control the flow of air within the chimney of sucha detector in such a way that air enters the combustion region only insufficient quantity to support combustion.

Another object of the present invention is to provide such a detector inwhich air in excess of that needed for combustion is exhausted from thechimney via a passage which is located so that currents of air adjacentthe combustion region are eliminated.

A further object is to provide an alkali flame ionization de' tector inwhich air sufficient for combustion is transported to the combustionregion solely by the forces of gaseous diffusion.

These and other objects, features and advantages of the presentinvention will become more apparent upon reading the following detaileddescription and examining the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of animproved AFID incorporating the means for altering the flow pattern inaccordance with the present invention; and

FIG. 2 is a graph illustrating the effect of hydrogen flow on theperformance of the AFID.

In FIG. 1, the improved AFID 1 according to the present inventionincludes a base 3, chimney 5 and a cap 7. The base 3 and chimney 5 maybe made of stainless steel while the cap 7 may be of aluminum, forexample. Each of these parts may be cylindrical about a vertical axis inthe drawing.

Mounted on base 3 is a jet assembly 9 from which emerge the gases to beburned. A connector assembly 11 provides a removable sealed couplingmeans for connecting the outlet end portion of a gas chromatographcolumn 13 in gas flow communication with jet assembly 9. A collectorelectrode assembly 15 and an igniter-polarizing voltage electrodeassembly 17 project through the wall of chimney 5 into the interiorthereof.

Jet assembly 9 comprises a hollow jet 19 which has near the lower end aconical sealing plug 21 which is joined to jet 19 as by swagging forexample. Plug 21 is received in a similarly conical recess in base 3 andforced into good sealing engagement with base 3 by an externallythreaded clamping nut 23. When nut 23 has been tightened there stillremains an annular recess 25 which interconnects an air flow passage 27and a series of bores 28 equispaced around the axis of nut 23. Passage27 is connected to a source of air under pressure (not shown) in orderto supply air to the interior of chimney 5 for combustion.

A salt cup 29 is mounted on the end of jet 19 and stored in place forexample by brazing. A duct 31 is aligned with the central bore in jet19. A salt of an alkali metal or alkaline earth metal 33, rubidiumsulfate (Rb S0,) for example, is pressed into an annular recess in cup29 under high pressure to form a solid block surrounding duct 31.

Chromatographic column 13 projects into a mixing chamber 34 and issealingly supported concentrically therein by an internally threadedsleeve 35 and a conical plug 37. A cylindrical adapter 39 is receivedwithin a counterbored recess in base 3 and may be sealed to base 3 byswagging for example. A hydrogen flow passage 41 interconnects mixingchamber 34 and a source of hydrogen gas under pressure (not shown). Ashort elbow-shaped duct 42 interconnects mixing chamber 34 and hollowjet l9.

Electrode assembly 17 comprises an igniter electrode 43 in the form of acoiled resistance heater element disposed adjacent the salt cup 29, apair of insulators 45 which enclose the leads for electrode 43, a shell47 which is fastened within an aperture in chimney for example bybrazing, and a two-terminal coaxial connector 49 which is insulatinglymounted in the shell 47.

Collector electrode assembly is similar except that only a singleinsulator and lead are needed to connect to a ringshaped collectorelectrode 51 disposed above the jet assembly 9. Although electrode 51 isactually disposed with the plane of the ring approximately perpendicularto the axis of symmetry of the chimney 5, it has been illustrated in thedrawing as if twisted about the axis of its electrical lead in order todisplay the circular configuration of the electrode.

Cap 7 comprises a plug 53 dimensioned to fit loosely within the bore ofchimney 5 and having an O-ring 55 disposed in a circumferential groovein plug 53 and providing a seal within chimney 5. A rod 57 disposed, forexample by press fitting, within a bore in plug 53 and a knob 59 ofphenolic for example, on the end of rod 57 form a convenient handle forremoval ofthe plug which becomes very hot in use.

In operation gaseous samples emerge serially from column 13 into mixingchamber 34. A controlled flow of hydrogen also enters chamber 34 aspreviously noted and flows into the annular space surrounding theportion of column 13 within chamber 34. The hydrogen then flows axiallywithin this annular space to the end of column 13 as shown by the arrowswhere it mixes with the effluent gaseous samples from column 13. Themixture then flows through elbow-shaped duct 42, into jet 19 and emergesfrom the duct 31 in salt cup 29.

Air to support the combustion of the gaseous mixture enters through airflow passage 27, flows into annular recess and upwardly through bores28. The air then emerges from the bores 28 as shown by the arrows intothe annular space surrounding jet 19.

With cap 7 removed to prevent explosion of the mixture of air andhydrogen, igniter electrode 43 is momentarily energized by a low voltageestablished between the leads from a power supply 61 to cause electrode43 to glow producing ignition of the combustible mixture. At this pointall of the air entering chimney 5 flows upwardly past the combustionregion and out the open top of chimney 5 together with combustionproducts as in the prior art detectors such as the one shown in theaforementioned US. Pat. No. 3,423,181.

With combustion underway the low voltage is removed from the leads toelement 43 and cap 7 is replaced, sealing the open end of chimney 7. Atthis point the flow pattern of air and combustion products changesradically. Since the top end of chimney 5 is this all gases must leavethe detector via the unsealed joint between chimney 5 and base 3. Themating surfaces at this point are not microscopically fiat and chimney 5may be held in place only by its own weight or, if desired, under lightpressure from a clamping device (not shown) such that smallinterconnected voids inevitably exist at the interface between chimney 5and base 3. These voids together comprise a radial flow passage ofrelatively small cross-sectional area around the base of chimney 5through which combustion products and air in excess of that necessaryfor combustion are vented to the outside atmosphere. Alternatively, oneor more small passages could be formed at the base of the chimney 5. Ifthis were done, base 3 and chimney 5 could be formed as one piece, whilesuch passages could be used to vary gas conductance by constricting orclosing one or more of them. Typically it has been found that a smallpressure on the order of 0.1-0.2 inches of water exists within theinterior of AFID l in operation due to the constricting effect of thesmall area radial flow passage described in the preceding paragraph. Apool or static mass of air is believed to exist in the regionssurrounding the top of nut 23 such that as air under pressure entersthis pool through bores 28, some air leaves through the above-describedradial flow passage while the remaining air travels upwardly to thecombustion zone above salt cup 29. As air is consumed by the combustionof gases, the partial pressure of oxygen in the region of the flame isreduced so that more air diffuses into the combustion zone from the poolof air below as shown by the arrows in the drawing.

As the mixture of hydrogen gas and the column effluent gases is burned,the salt in salt cup 29 is heated. The effluent vapors of hydrocarboncompounds burning in the presence of this heated salt mass form aconductive plasma of ions in the interelectrode space between electrodes43 and 51. The presence of these positive and negative ions in theinterelectrode space lowers the resistance of this space. A polarizingvoltage of, for example, 300 volts DC is applied in the series circuitloop which includes power supply 61, the input of an electrometeramplifier 63, collector electrode 51, igniterpolarizing electrode 43 andthe associated leads, with the igniter-polarizing electrode typicallybeing negatively polarized, and the collector electrode being at or nearground. The current which flows in the circuit is proportional to thedensity of ions in the interelectrode space.

When the effluent gas from column 13 consists ofonly pure carrier gaswithout any of the gaseous components to be detected, a relativelysteady background current exists in the series circuit. Since thisbackground current contains no information about the samples of interestit is desirable to, in effect, subtract it from the current to bemeasured in amplifier 63 and the remaining circuitry. For this purpose abucking current source 65 passes an adjustable current through the inputof amplifier 63 in opposition to the main current flowing from theinterelectrode space. Source 65 can be adjusted to null any backgroundcurrent so that only current indicative of a sample gas will beregistered.

After amplification in amplifier 63 the signal output is connected,through an attenuator 67, to a recorder 69 which is typically of thestrip chart type. A range switch 71, which may consist of a bank ofselectable feedback resistors, is connected from output to input ofamplifier 63 in order to adjust the amplifier again.

Although a particular electrometer circuit has been discussed in theforegoing, it should be understood that other electrometers could beused as well without affecting the performance of the AFID. See, forexample, Littlewood, Gas Chromatography, pages 289-292, Academic Press,1962.

Turning now to FIG. 2 a graph summarizing performance factors andillustrating the optimization of operating conditions is shown. The AFIDused to 12 this data was as shown in FIG. 1 and had the followingapproximate dimensions: Chimney 5 height2 inches; inside noise, ofchimney 5-seveneighths inch; height of collector electrode 51 above topof salt cup 29five-eighths inch; loop diameter of collector electrode5l--one-fourth inch; distance from top of salt cup 29 to top of base3-eleven-sixteenths inch. The ignitor electrode 43 was held at apotential of 300 volts below collector electrode 51 potential and thesalt used in salt cup 29 was Rb S0,. The carrier gas flowing throughcolumn 13 was helium.

As noted in FIG. 2, an air flow of 230 ml./min. was chosen sinceinvestigations have shown that flow rates between 200 and 270 ml./min.yield optimum results with this detector. Then the hydrogen flow wasadjusted over a range of to 20 ml./min. starting at 85 ml./min.

The solid line of FIG. 2 indicates that background current reached amaximum of somewhat more than 5X10 amps at approximately 43 ml./min. ofhydrogen. Since high background currents correspond to rapid consumptionof alkali salt and to excessive noise, operation of the detector at 43ml./min. of hydrogen is plainly undesirable in this case.

The response curve which is plotted only qualitatively with respect tothe ordinate scale passes through a maximum at about the same hydrogenflow of 42-43 ml./min. However response which may be measured incoulombs of electric charge crossing the interelectrode space per gramof sample which, when burned, produces this charge transfer, does nottake into account the noise level.

Hence in determining the optimum hydrogen flow, sensitivity" which isequivalent to signal-to-noise ratio should under most circumstances bemaximized. FIG. 2 shows that sensitivity, which may be defined as theminimum sample flow rate in grams per second which can be reliablydetected, reaches a maximum at approximately 37 mL/min. of hydrogen. Atthis hydrogen flow, background current is only 50 percent of the maximumvalue. Qualitatively similar results have been produced using a varietyof AFlDs constructed according to FIG. 1 although the optimum air andhydrogen flows have been found to vary slightly according to the fit ofthe chimney 5 to the base 3 and the exact positioning of the electrodes.In general then the procedure for establishing optimum operatingconditions is to choose an air flow in the range of 200 to 270 ml./min.and to scan hydrogen fiow in step changes from 85 to 20 ml./min. toproduce a curve of background current similar to FIG. 2. The hydrogenflow should then be set at the value corresponding to 50 percent ofmaximum background current on the rising portion of the backgroundcurrent curve.

However, it has also been discovered that higher air flows yield highermaximum background currents and higher noise levels as a result. As ageneral rule maximum background currents in excess of approximately 3x10amp are an indication that a lower air flow should be chosen in order tominimize noise level.

The performance of the AFlD according to FIG. 1 has been evaluated usingas alkali salts KBr, RbBr, CsBr, K 50, and Rb,SO Each required differentH and air flows for optimum performance. With each of these salts theAFID exhibited very high sensitivity to nitrogenandphosphorous-containing hydrocarbon compounds compared to otherhydrocarbon compounds i.e. the selectivity for the nitrogen andphosphorous compounds was excellent such that they could be reliablydetected and quantified even in the presence of other hydrocarboncompounds in the effluent gases from the column. The AFlD also exhibitedgood selectivity for sulfurand chlorine-containing compounds.

When the AFlD according to the present invention was compared with thedetector according to U.S. Pat. No. 3,423,181, it was discovered thatbackground current had been decreased from about 3X10" amp to 3X10 ampor a factor of l00. Similarly noise level had dropped from ZXIO" amp to2X10 amp. At the same time the sensitivity to phosphorous-containingcompounds had improved by approximately 20 times.

What is claimed is:

1. A chromatographic detector for analyzing the gaseous components inthe effluent gas emerging from a chromatographic column comprising incombination: an enclosure defining at one end thereof an open portion; acap means for sealingly closing said open portion; a hollow jet memberextending into said enclosure from one wall thereof, said jet memberhaving an open end within said enclosure; said enclosure defining afirst gas flow passage in communication with said jet and with theexterior of said enclosure for connection to said chromatographiccolumn; means in communication with said passage for mixing saideffluent gases with a combustible gas to form a combustible mixture;said enclosure defining a second gas flow passage therethroug'h forconducting an oxygen-containing gas mixture to the interior of saidenvelope from a source of said oxygen-containing gas mixture; meansdisposed adjacent said open end of said jet member for producing ions inresponse to the combustion of said combustible gas mixture; saidenclosure defining a third gas flow passage extending from the exteriorof said enclosure to portrons of the interior of said enclosure distalsaid open portion thereof for exhausting products of combustion from theinterior of said enclosure; first and second electrodes insulatinglyextending through a wall of said enclosure and defining between the endsthereof within the interior of said enclosure an interelectrode gap;means for causing an electrical current to flow across saidinterelectrode gap and means for measuring the magnitude of saidelectrical current.

2. A detector according to claim 1 wherein said enclosure includes abase portion defining on one surface thereof a recess, and a chimneyportion having one end thereof received in nesting relation within saidrecess and wherein said third gas flow passage is defined between themating surfaces of said base portion and said chimney portion.

3. A detector according to claim 1 wherein said cap means comprises aplug dimensioned to be insertable within said open portion of saidenclosure and having a resilient gasket extending around the peripherythereof.

4. The detector according to claim 1 wherein one of said electrodesincorporates means for igniting said combustible gas mixture.

5. The detector according to claim 1 wherein said means for producingions comprises a cup member surrounding said open end of said jetmember, said cup member containing an alkali salt.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION September 21, 1971Patent No. 3 607 096 Dated Inventor(s) Charles H. Harmann It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

After the name and address of the inventor --Assignee: Varian AssociatesPalo Alto, California-.

Insert:

Signed and sealed this 13th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officerand Trademarks

2. A detector according to claim 1 wherein said enclosure includes abase portion defining on one surface thereof a recess, and a chimneyportion having one end thereof received in nesting relation within saidrecess and wherein said third gas flow passage is defined between themating surfaces of said base portion and said chimney portion.
 3. Adetector according to claim 1 wherein said cap means comprises a plugdimensioned to be insertable within said open portion of said enclosureand having a resilient gasket extending around the periphery thereof. 4.The detector according to claim 1 wherein one of said electrodesincorporates means for igniting said combustible gas mixture.
 5. Thedetector according to claim 1 wherein said means for producing ionscomprises a cup member surrounding said open end of said jet Member,said cup member containing an alkali salt.